Typically, in a die bonding process, each die is picked up by a pick head of a die bonder from a wafer and then transported to a substrate where it is bonded onto the substrate. In order to place the die correctly and accurately onto the substrate, visual alignment is performed to capture images of the die on the wafer platform and of the substrate respectively. A die bonder usually incorporates a first optical system to capture an image of the die before the bond head is moved into position to pick up the die while a second optical system captures an image of the die bonding site on the substrate. The orientation of the die is adjusted based upon the image taken of the die bonding site, before the die is placed onto the substrate. A problem with this approach is that motion error may unintentionally result during the die pick-up process after the image of the die has been captured. This error cannot be corrected or compensated for by the vision system. Moreover, when a die is lifted from an adhesive Mylar paper on which it is mounted, an unexpected die offset or rotation relative to the pick head may arise when the die leaves the film surface.
US Patent Publication No. 2007/0134904 A1 entitled “High Precision Die Bonding Apparatus” discloses a method to reduce motion error by determining the position and orientation of the die only after it has been picked up from the wafer platform. The orientation of a die being held by a pick-up tool between an optical assembly and a die bonding site is viewable by the optical assembly so as to align the orientation of the die with the orientation of the die bonding site before the die is bonded to the substrate. Thus, motion error due to the pick up process of the die is avoided. However in this prior art, the type of dice which can be aligned using this method is limited to long dice, and the viewable areas of the die are limited to the boundary of the die, especially the corners of the die as the view of the center of the die is blocked by the pick-up tool. Furthermore, the die must have fiducial marks at the corners for alignment.
FIG. 1 is a schematic illustration of a conventional method of visually aligning a die 102 with a bonding site on a bonding substrate 104 to rectify motion error after picking up the die 102. In the die bonding process, a first alignment optical system 106 captures an image of the die 102 located on a wafer platform 108 before the die 102 is picked up by a pick head 110. The image of the circuit pattern which resides on the top side of the die 102 is captured. The die 102 is next aligned using a boundary of the die from the bottom side of the die 102 by a second alignment optical system 112. Thereafter, the alignment of the die 102 may be further adjusted after imaging the bonding site with a third alignment optical system 114 located over a bonding site at the bonding substrate 104 by aligning the die relative to the bonding substrate 104. This method of alignment however requires complex calibration of the die pattern and the die boundary. Furthermore, it may be unreliable to conduct alignment based on the die boundary since accuracy of the alignment based on the boundary of a die alone is subject to the sawing quality of the die and the conditions of the bottom side of the die. When the edge is not sufficiently straight due to poor cutting of the die, the boundary of the die would not be accurately defined for accurate alignment.
FIG. 2 shows a schematic illustration of another conventional method of visually aligning the die 102 with a bonding site on a bonding substrate 104 to rectify and reduce motion error after picking up the die 102 by using an intermediate stage 116. After the die 102 has been picked up by the pick head 110, the die 102 is placed on the intermediate stage 116 and it is imaged by an intermediate alignment optical system 118. The circuit pattern on the top side of the die 102 is viewable and is imaged for aligning the die 102. Next, the die 102 is picked up again and placed on a bonding site on the bonding substrate 104 after the die position relative to the bonding substrate 104 has been aligned by the third alignment optical system 114. The process of picking up the die 102 from the alignment or intermediate stage 116 should introduce less positional error than when the die 102 is picked up directly from a wafer platform 108 since the orientation of the die 102 when it is on the alignment or intermediate stage 116 can be imaged again for alignment before it is picked up for die bonding and the adhesion effect of the Mylar paper in the pick-up process is avoided in this approach. Hence, the positional error is reduced.
However, this method of aligning a die requires an additional operation which makes the machine more complicated and reduces the machine throughput speed. Though error due to the adhesion force of Mylar paper is avoided, this prior art still encounters positional errors from the pick up operation of the die from the intermediate stage 116. An example of a prior art which uses an intermediate or an alignment stage is U.S. Pat. No. 6,321,971 entitled “Die Collet For A Semiconductor Chip And Apparatus For Bonding Semiconductor Chip To A Lead Frame”.
In another bonding application, inspection is commonly carried out for checking the accuracy and quality of flip-chip bonding. FIG. 3 is a schematic illustration of a conventional inspection method for inspection of placement accuracy of a flip-chip 102′. The flip-chip 102′ has been transferred from a wafer platform 108 to a bond site on the bonding substrate 104 where bumps 120 on the underside of the flip-chip 102′ are bonded to bond pads 122. The third alignment optical system 114 may be deployed to capture the image of the flip-chip 102′ and align the boundary of the flip-chip 102′ with the bonding substrate 104 for checking the accuracy and quality of bonding. This method of checking bonding accuracy using the boundary of the flip-chip 102′ is however not accurate since the alignment of the circuit pattern rather than the boundary of the flip-chip is of interest. The relationship between the die boundary and the circuit pattern may also not be straightforward. Additionally, the die boundary may not be well-defined due to poor sawing quality which results in uneven edges. Rough edges or cracks along the edge may also give unreliable results if one checks alignment using the die boundary.
Further, post-bond inspection may entail checking epoxy adhesive such as silver epoxy under an integrated circuit die and under-fill glue beneath a flip-chip. However, the epoxy adhesive or the under-fill glue is not viewable using a conventional optical system. Generally, the adhesive and the under-fill are checked by off-line X-ray equipment or by ultrasonic imaging equipment. Furthermore, post-bond inspection of flip-chip bonding on the degree of displacement of the circuit pattern (for example, displacement of metal traces and bond pads) with respect to the bonding substrate is not feasible as the die is flipped. The die position can only be estimated from the die boundary. It is therefore advantageous to make available an on-line post-bond checking method for the coverage of under-fill glue or silver epoxy adhesive under the die as well as the circuit pattern of a bonded flip-chip, where such post-bond checking may be performed on the same machine or on a different machine. It would also be desirable to perform visual alignment of a die after it has been picked by the pick head and before it is placed onto a bonding substrate to avoid the aforesaid motion error and adhesion error introduced when the die is being picked up.